GRAYLING OF GREBE LAKE 



349 



SUMMARY 



The grayling, Thymallus arcticus, is common 

 both to North America and to eastern Asia. In 

 the United States the species is highly prized by 

 sportsmen because of its great beauty and com- 

 parative rarity. The only indigenous population 

 maintaining itself at present in the United States 

 is in the Red Rock Lakes region in Montana. 

 From this center, it was introduced in other 

 waters, including some streams and lakes in 

 Yellowstone National Park, Wyo. 



The grayling was first planted in Grebe Lake 

 of the Park in 1921, 14 years after the rainbow 

 trout had been introduced and 9 years after the 

 cutthroat trout had been planted there. The two 

 species of trout intermixed after planting and by 

 1952 only hybrids were found. The grayling re- 

 portedly has experienced wide fluctuations in 

 population abundance at intervals since its intro- 

 duction. One such decline in numbers in 1949 

 resulted in the present study on Grebe Lake. 

 The following statements summarize the findings 

 of my work between July 22, 1952, and September 

 1, 1954. 



Grebe Lake is eutrophic with a well-developed 

 thermocline during the period of summer stagna- 

 tion and a deficiency of oxygen in the deep waters 

 of 1953 and 1954. Samples of the aquatic fauna 

 in Grebe Lake showed a predominance of Tendi- 

 pedidae ( = Chironomidae) both numerically and 

 volumetrically. 



Population estimates in 1953 and 1954 were 

 obtained from trap nets and a mark-and-recapture 

 method. About 27,000 grayling were in Grebe 

 Lake in 1953 and 26,000 in 1954. Trout approx- 

 imated 2,000 the first year and 2,500 the second. 

 The standing crop of fish in Grebe Lake in 1953 

 was estimated at 77 pounds per acre: about 70 

 pounds per acre of grayling and 7 pounds per 

 acre of trout. In 1954, the standing crop was 

 approximated at 71 pounds per acre of grayling 

 and 8.3 pounds of trout. 



Both the grayling and the hybrid trout spawned 

 in all four tributaries and in the outlet of Grebe 

 Lake between mid-May and late June. Stream 

 temperatures at this time ranged from 40° F. to 

 57° F. Weirs or blockades installed in the water- 

 ways during early May to count spawning mi- 

 grants provided a means for enumeration of 7,081 

 grayling and 674 trout in 1953 and 7,878 and 405, 

 respectively, in 1954. 



Length-frequency polygons of the grayling 

 spawners showed that females averaged smaller 

 than the males. Lengths of the hybrid trout were 

 variable and no strong modes were evident when 

 lengths were plotted against numbers. 



Males of the grayling outnumbered the females 

 in early spawning runs each year. The ratios 

 assumed a more nearly 1-to-l proportion as the 

 seasons progressed. For this species overall sex 

 ratios in Grebe Lake during 1953 were 10 females 

 to 24 males and, in 1954, 10 females to 9 males. 



The natural fry production in Grebe Lake in 

 1954 was approximated as 236,500 or 2.5 percent 

 of the estimated number of eggs produced by the 

 spawning adults. The greatest mortality during 

 this time was attributed to dislodgment of the 

 eggs during the incubation period either by subse- 

 quent spawners or by changes in water level and 

 current velocity. 



Scales for age-and-growth analyses were col- 

 lected from both the grayling and the hybrid trout 

 throughout the study period. The time of annulus 

 formation for grayling younger than 3 years was 

 found to be prior to June 25. No scales from 

 3-year-olds less than 13 inches in total length had 

 an evident annulus during the spawning season. 

 The interpretations of scales from the grayling 

 more than 3 years of age were considered inaccurate 

 because of the small annual growth in body and 

 scale lengths which in some samples either had 

 obscured the correct interpretation of existing 

 marks or had precluded annulus formation entirely. 

 An additional complication often resulted from a 

 failure of annulus I to form on individuals which 

 had been too small to have had scales at the start 

 of their second growing season. 



None of the grayling matured before reaching 

 age-group III. The legal size of 6 inches was at- 

 tained by all during their second summer of life. 

 Averages of the calculated total lengths in inches 

 for the grayling at the end of their first 3 years of 

 life were: I, 4.4; II, 8.9; III, 11.2. 



Females of the trout grew faster than did the 

 males and had matured at the start of their fourth 

 summer of life (age-group III). Some mature 

 males of age-group II appeared in the spawning 

 runs. The legal size limit of 6 inches was reached 

 either in the second or the third year of life. Some 

 trout were also too small to have had scales at the 

 end of their first summer and consequently lacked 

 their first year mark. The average calculated total 



